I have an Allison boat with a 225 Mercury Engine, Minn Kota 36 volt trolling motor, a single 12 volt crank battery and three 12 volt marine batteries wired to 36 volt for trolling.
Recently installed onboard Stealth Charger DC/AC. It allows the engine alternator to charge the crank battery and the Stealth Dc unit converts to 36V and charges the trolling batteries. The AC charger tops off boat batteries while boat is stored. All four batteries are good. Both the AC and DC charging works correctly.
Since installing the charger I have run the engine about six hours and trolling motor about fifty hours. On the first trip out, it was one hour on the engine and six hours trolling.
I noticed the trolling motor stainless steel prop nut rusted and my stainless steel engine prop corroded and the unpainted Sportmaster lower unit housing corroded. I had been on a lake I normally don’t go to. I thought it was something in the water.
Well, I just got back from four days of fishing – about five hours on engine and 44 hours trolling. The heads of the stainless steel screws that hold the transducer in the trolling motor had completely rusted off and water went in the trolling motor and the motor quit. The skeg and motor housing appear to be powder coated and the powder coat was bubbled up and white corrosion on housing and skeg.
My dealer tells me that I have a boat wiring problem and I had also probably hit a stump with trolling motor because the skeg was not vertical with trolling motor shaft.
The best that I recall the skeg was never vertical with the shaft. So I took the motor apart the 10 inch long bolts were not bent. There is a channel if bolts were installed correctly the bolts would be in the middle of the space between the motor magnets and the skeg would be vertical. The bolts were over close or against the magnets. The water ingestion and continuing to run caused the motor to fail.
My question: Is the trolling motor, charger, or boat wiring causing the extreme corrosion? I checked and all of my boat battery wiring is clean and tight.
Galvanic corrosion occurs when you have two different metal objects that are electrically connected and immersed in the same body of electrolyte. Basically, the part that is more + on a DC meter will corrode away to protect the item that is more -.
You try to keep this difference down below 0.5 volts to keep metals from corroding quickly. When you accidentally apply 12 volts (or maybe even 36 volts in your case) to one of the metal objects in this cell, it will corrode very quickly.
My best guess: 12 volts or higher was accidentally applied to the exterior metal of the trolling motor housing (motor short, transducer housing, transducer cable, trolling motor connection, etc). When the charger was installed, it completed the galvanic cell to allow the corrosion to begin.
Thanks for the quick response. Since I e-mailed you I talked to the owner of Stealth Chargers.
He said what changed when I installed the charger is we now have a common ground between the crank & trolling batteries. Prior to charger install the trolling motor + & – were independent.
The Minn Kota trolling motor assembled with the case bolts against the magnets is probably the voltage leak. Add the common ground and boom you get a floating battery that as been known to bring fish to the surface. He told me how to check it once I replace the trolling motor to make sure it was not coming from another source. I’ll let you know how it turns out.